This invention relates to a process for cost-effectively producing commercial products from natural gas. More particularly, this invention relates to a combined process for producing a liquefied natural gas (LNG) product, a crude helium, and a synthesis gas.
Natural gas generally refers to rarefied or gaseous hydrocarbons found in the earth. Non-combustible natural gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases.
Natural gas is often plentiful in regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
It is common practice to cryogenically liquefy natural gas so as to produce liquefied natural gas (LNG) for storage and transport. A fundamental reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers at low or even atmospheric pressure. Liquefaction of natural gas is of even greater importance in enabling the transport of gas from a supply source to market where the source and market are separated by great distances and pipeline transport is not practical nor economically feasible.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. (−151° C.) to −260° F. (−162° C.) where it may exist as a liquid at near atmospheric vapor pressure. Various systems exist in the prior art for liquefying natural gas or the like whereby the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages, cooling the gas to successively lower temperatures until liquefaction is achieved. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, nitrogen, and methane, or mixtures thereof. The refrigerants are commonly arranged in a cascaded manner, in order of diminishing refrigerant boiling point.
Additionally, chilled, pressurized natural gas can be expanded to atmospheric pressure by passing the natural gas through one or more expansion stages. During the course of this expansion to atmospheric pressure, the gas is further cooled to a suitable storage or transport temperature by flash vaporizing at least a portion of the already liquefied natural gas. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or burned to generate power for the LNG manufacturing facility.
LNG projects have not always been economical in that cryogenic refrigeration systems are highly energy intensive and require a substantial capital investment. In addition, participating in the LNG business requires further investment for sophisticated and costly shipping vessels and regasification systems so that the LNG consumer can process the product.
An alternative to the cryogenic liquefaction of natural gas to LNG is the chemical conversion of natural gas into products, for example Gas-To-Liquid (GTL) products via the production of synthesis gas (syngas). Synthesis gas is herein defined as a gas comprising hydrogen and carbon dioxide. A synthesis gas generator is herein defined as any device that produces synthesis gas as an intermediate or final product.
Traditional GTL products include, but are not limited to, methanol, acetic acid, olefins, dimethyl ether, dimethoxy methane, polydimethoxy methane, urea, ammonia, fertilizer, Fischer Tropsch reaction products, and hydrogen. The Fischer Tropsch reaction produces mostly paraffinic products of varying carbon chain length, useful for producing lower boiling alkanes, naphtha, distillates useful as jet and diesel fuel and furnace oil, and lubricating oil and wax base stocks.
The most common commercial methods for producing synthesis gas are steam-methane reforming, auto-thermal reforming, gas heated reforming, partial oxidation, and combinations thereof. Emerging technologies include catalytic partial oxidation and ion transport membrane (ITM) processes.
Steam methane reforming generally reacts steam and natural gas at high temperatures and moderate pressures over a reduced nickel-containing catalyst to produce synthesis gas.
Autothermal reforming generally processes steam, natural gas and oxygen through a specialized burner where only a portion of the methane from the natural gas is combusted. Partial combustion of the natural gas provides the heat necessary to conduct the reforming reactions that will occur over a catalyst bed located in proximity to the burner.
Gas heated reforming consists of two reactors or reaction zones, a gas heated reformer reactor/zone and an autothermal reformer reactor/zone. In one configuration, steam and natural gas are fed to the gas-heated reformer where a portion of the natural gas reacts, over catalyst, to form synthesis gas. This mixture of unreacted natural gas and synthesis gas is then fed to the autothermal reformer, along with oxygen, where the remaining natural gas is converted to synthesis gas. The hot synthesis gas stream exiting the autothermal reformer is then routed back to the gas reformer to provide the heat of reaction necessary for the gas-heated reformer.
Partial oxidation reforming generally processes natural gas, oxygen, and optionally steam through a specialized burner where a substantial portion of the methane is combusted at high temperatures to produce synthesis gas. In contrast to autothermal reforming, no catalyst is present in the partial oxidation reactor.
Current technology for manufacturing synthesis gas is highly capital intensive. Autothermal and partial oxidative synthesis gas methods generally require a costly air separation plant to produce oxygen. Steam reforming, which does not require oxygen manufacture, produces a synthesis gas having a higher hydrogen to carbon monoxide ratio that is less than stoichiometrically optimum for manufacture of Fischer Tropsch products. Additionally, the market for GTL products such as dimethyl ether and Fischer Tropsch products has been erratic or in some cases, insufficiently established to overcome the substantial capital investment risk inherent in erecting such plants.
Natural gas reserve holders have found that substantially increasing the capacity of a LNG or GTL plant can improve plant construction economics. Many of the costs inherent to building such plants are fixed or minimally, do not increase linearly with capacity. However, it has also been found that as more of a single product is produced in a distinct and often isolated geographical region, the product price over cost margin is reduced.
Helium is increasingly in demand for several applications, e.g. as a shielding gas during welding and in the chemical industry, coolant for MRI magnets, as a quenching gas in metals processing, as inert gas in space technology, as a respiration gas during diving, as a carrier gas in chromatography, for the detection of leakages, as a balloon-filling gas and for other purposes as well. For these purposes, high purity helium may be required. In order to achieve high purity helium from gas mixtures containing only low levels of helium, several processing steps may be required. The gas mixture may be processed to form a crude helium gas mixture and subsequently purified to form a high purity helium stream from this crude helium gas mixture.
Helium is enriched and recovered mainly from helium-containing natural gases. The main components of these natural gases are nitrogen and methane, as well as up to 10% helium by volume, besides lower proportions of several higher-molecular weight hydrocarbons and carbon dioxide.
Helium typically occurs in very low concentrations in certain natural gas fields. Natural gas streams from which helium can be economically recovered typically contain at least approximately 0.1 volume % to 0.5 volume % helium. This helium may be upgraded to produce a crude helium containing typically at least 20 volume % helium. Crude helium is defined herein to be a fluid containing greater than 20 volume % helium.
Methods of helium enrichment are known.
The helium-containing natural gas is cooled down to approximately −150° C. in a cryogenic plant, whereby primarily the hydrocarbons will be separated out by condensation. The so-produced gas mixture, except for low proportions of other gases may contain more than 50% by volume helium and nitrogen. Such crude helium can be treated on site to give a helium of very high purity, e.g. by subjecting it to some process combination comprising a pressure swing adsorption plant and a second cryogenation unit.
Another alternative is to sell the crude helium as an intermediate product to be treated by a third party.
The present invention is useful for recovering helium from natural gas reserves. It would be desirable to effectively combine crude helium, LNG, and synthesis gas production. It would be desirable to recover helium from natural gas reserves where the helium concentration is less than 0.1 volume %. Heretofore, helium recovery from low helium concentration natural gas reserves was considered not viable.
The current invention satisfies the growing need for helium for industrial processes and growing need for fossil fuel energy, especially clean burning fuels.